1. Field of the Invention
The present invention relates generally to a system for determining the angular position of a radar target by using an electronically scanned, linear phased array radar antenna system, and more particularly pertains to an elevation determining system as described designed to utilize a conformal radar antenna mounted in an aircraft and formed by a linear array of endfire elements.
The subject invention is described and illustrated herein with respect to a particular embodiment of an airborne radar system on an aircraft for determining the elevation of a radar target with respect to the aircraft. However, the teachings of the present invention are also applicable to embodiments of ground or sea based radar systems for determining the elevation of an airborne target, and to embodiments for determining the azimuth of a radar target with respect to the radar system.
2. Discussion of the Prior Art
Ganz, et al. U.S. Pat. No. 4,336,543 discloses an electronically scanned antenna system having a linear array of endfire elements of the general type for which the elevation determining system of the present invention was developed. In this antenna system, the endfire elements are laterally spaced between about 0.32 .lambda. to 0.92 .lambda. apart, preferably about 0.552 .lambda. apart, to enhance the effects of mutual coupling therebetween for broadening the radiation signal pattern of the elements in the plane of the array. The endfire elements may be of the Yagi type with each endfire element including a common reflector, a driver, and a plurality of directors. This provides an antenna array of very small vertical dimension so as to be suitable for conformal installation on or within the airfoil surfaces of an aircraft, e.g., wing leading edges and the horizontal stabilizer trailing edge. Such a phased array antenna system can offer surveillance and missile guidance capabilities with high levels of jam resistance at reduced weight, power and volume.
As is known in the art, in an electronically scanned antenna having an array of transmit/receive elements, electronic scanning entails adjustment in the phase excitation coefficients of the elements in the array in accordance with the direction in which the formation of the transmitted beam is desired. It is well known that the beam of an antenna points in a direction that is normal to its phase front. In phased array antennas, the phase front is adjusted to steer the beam by individual control of the phase excitation of each radiating element, which is known in the art as antenna feed. Phase shifters are electronically actuated to permit rapid scanning and are adjusted in phase to a value between 0 and 2 .pi. radians.
One important function of many radar systems is the accurate determination of target position. Target position information is essential in such diverse applications as air-traffic control, antiaircraft artillery direction, missile guidance, and satellite tracking. Target azimuth and elevation may be determined by using conventional scanning antennas and appropriate scanning formats. However, when accurate target position and motion information is desired, special radars called tracking radars are often used. Some of the more common tracking radar systems use conical scan, sequential lobing, or phase or amplitude monopulse techniques to sense target position. Each of these techniques involves the use of information obtained from offset antennas or antenna beams to develop signals related to the angular errors between the target position and the boresight axis of the tracking antenna.
The use of monopulse signal processing techniques is a significant feature of the present invention. In such radar systems, information on target direction is obtained from two simultaneous overlapping beam patterns. From these two patterns, sum and difference patterns are generated. The difference pattern is formed by subtracting the received voltage in the first pattern from that of the second pattern, and the sum pattern is formed by adding the received voltages in the first and second patterns. In a simplified monopulse radar system for tracking in one angular coordinate, the signal is transmitted through two apertures by a hybrid junction, and the two received signals are fed to a "magic tee" or similar junction that forms the sum (S) and difference (D) signals, which are then amplified in two identical channels. The difference signal indicates the magnitude of the error, but the error direction is ambiguous. The ambiguity is resolved by comparing the phase of the difference signal with the phase of the sum signal.